The present invention relates to a circulating fluidized bed reactor in accordance with the pending claims.
In more detail, the invention relates to a circulating fluidized bed reactor, comprising a furnace, the lower part of which is provided with fluidizing gas nozzles for fluidizing bed material to be fed to the furnace, said furnace being defined by a substantially vertical and planar first wall; a particle separator for separating bed material from the gas discharged from the reactor; a return duct for bed material separated in the particle separator, arranged in connection with said first wall and having a lower part; a gas seal arranged in the lower part of the return duct, preventing gas from flowing from the furnace to the return duct; and a receiving space defined by a planar water tube wall, which receiving space may be said furnace, whereby the water tube wall is the first wall, or a space in gas flow connection with the furnace.
It is generally known to manufacture a gas seal of a loop seal type, an L-seal or a seal pot for the return duct of a circulating fluidized bed reactor. In all these cases, the return duct of the separator comprises a duct or a section filled with bed material circulating from the particle separator to the furnace, thus preventing furnace gas from flowing via the return duct to the separator. In conventional separator arrangements, the return duct is uncooled and apart from the furnace wall, wherefore, it has also been natural to arrange the gas seal to be an uncooled construction spaced apart from the furnace wall. It is inevitable, however, that joining uncooled structures to a cooled furnace results in temperature differences and thermal stresses reducing the durability and reliability of the equipment.
Published European patent document 0 082 673 discloses an uncooled gas seal vessel integrated in the wall of the lower part of an uncooled furnace. However, the disclosed arrangement is heavy, extending considerably far from the furnace, and, therefore, needs to be thoroughly supported. Furthermore, uncooled structures can easily get broken due to temperature differences, especially during the start-up and shutdown of the reactor.
U.S. Pat. No. 4,951,612 discloses a fluidized bed boiler having four separate gas seals integrated in the cooled outer wall of a cylindrical furnace. The structure of the gas seals is, however, not illustrated in detail.
U.S. Pat. No. 5,269,262 discloses a cylindrical fluidized bed boiler, having a cylindrical structure in the middle thereof, said structure comprising a particle separator, return duct and a multipart, partly cooled gas seal. In the given arrangement, the durability of the furnace wall reduces considerably at the return openings for circulating material and the wide solid wall surfaces between the openings interfere with the even distribution of the material in the furnace.
U.S. Pat. No. 5,281,398 discloses a new kind of a cooled particle separator for a circulating fluidized bed reactor with a cooled return duct integrated in the cooled wall of the furnace. Especially, in this kind of arrangement, it is advantageous to have a cooled gas seal arranged to communicate with the furnace wall. U.S. Pat. No. 5,341,766 discloses a gas seal of a gill seal type meeting said requirements, which gas seal comprises a number of narrow gaps and is integrated directly in the furnace wall. Practice has proved that the usability of a gas seal of a gill seal type is generally good, but in some special situations, its operation capacity may decrease.
U.S. Pat. No. 5,526,775 discloses a gill seal type gas seal between a return duct and the upper part of a heat exchange chamber, which heat exchange chamber is closely connected to a reactor chamber wall. The heat exchange number is in flow communication with the reactor chamber through a vertical discharge channel and one or more openings. U.S. Pat. No. 4,716,856 discloses a heat exchange chamber arranged in a bent wall section of a reactor, where a return duct leads hot material in a fluidized bed in the heat exchange chamber.
An object of the present invention is to provide a method and an apparatus, in which the above mentioned problems of the prior art have been minimized.
It is especially an object of the present invention to provide a circulating fluidized bed reactor, which has a space-saving gas seal integrated in the planar cooled boiler wall, without reducing the bearing capacity thereof.
Further, an object of the invention is to provide a circulating fluidized bed reactor, which has a light, durable and reliable gas seal.
It is also an object of the present invention to provide a circulating fluidized bed reactor, in which the distribution of the bed material recycled from the gas seal has been improved in the direction of the wall of the receiving space.
In order to achieve these objects, a circulating fluidized bed reactor is provided, the characterizing features of which are discussed in more detail below.
Thus, it is a characteristic of the circulating fluidized bed reactor in accordance with the present invention that a gas seal is arranged in connection with a water tube wall defining a receiving space in such a way that the horizontal cross-sectional width of the lower part of the return duct measured in the direction of the first wall is larger than the depth perpendicular to said width, and the gas seal has a seal structure comprising water tubes joined to each other and being formed by bending water tubes from the water tube wall defining the receiving space.
In a simple case, the lower part of the return duct of the separator is in direct connection with the furnace, whereby, according to the present invention, a gas seal may be arranged in connection with the furnace wall. In some cases, however, the return duct is joined to the furnace via a separate heat exchange chamber in such a way that the heat exchange chamber is in gas flow connection with the furnace and the gas seal is arranged upstream of the heat exchanger. In this case, a gas seal in accordance with the present invention may be formed in connection with the wall of the heat exchange chamber, which is in gas flow connection with the furnace.
It is apparent to those skilled in the art that a gas seal in accordance with the present invention may also be arranged in connection with another comparable cooled wall defining a space in gas flow connection with the lower part of the furnace. The present invention is described below in more detail in connection with the furnace wall, but it is to be understood that the description above also involves gas seals in connection with the walls of other spaces in gas flow connection with the furnace of a circulating fluidized bed boiler.
The gas seal in accordance with the present invention preferably comprises at least one seal channel arranged in the lower end of the return duct, said channel being defined by a front wall and a seal structure, which separates a distinct portion from the bed of circulating material being formed in the lower part of the return duct. The seal channel is preferably in flow connection with the return duct only at the lower part of the seal structure, and only at the upper part of the front wall in flow connection with return means formed in the water tube wall defining the furnace.
When the lower edge of the means joining the seal channel to the furnace, i.e., the return means, is located higher up than the upper edge of the means joining the seal channel to the return duct, the seal channel comprises a center part, which is in a horizontal direction totally surrounded by walls, and a bed of circulating material is formed in the seal channel. The bed surface is substantially flush with the lower edge of the return means. Thus, the bed material in the seal channel prevents gas from flowing from the furnace to the return duct.
In order to make the bed material flow from the return duct through the seal channel to the furnace, the bed material in the seal channel is preferably fluidized by means of fluidizing gas, which is supplied through fluidization gas nozzles arranged in the lower part of the seal channel. Due to fluidization, the bed surface lies typically somewhat higher up in the seal channel than outside the seal channel in the lower part of the return duct. On the other hand, the friction caused by the bed material flow and the pressure difference prevailing between the furnace and the return duct tend to raise the bed surface in steady state conditions in the lower part of the return duct outside the seal channel.
In such cases where the fluidization of the seal channel is not necessary or it is very slow, the bed surface in the seal channel may be slightly inclined towards the front wall, whereby the gas lock is tight, even if the lower edge of the return means is approximately flush with or even slightly lower down than the upper edge of the means connected to the return duct.
Preferably, the seal structure comprises a side wall in connection with the front wall, said side wall being cooled by means of water tubes bent from the wall defining the furnace. Thereby, the water tubes may form a supporting structure for the side wall at the same time supporting the furnace wall and preventing return means formed on the wall from weakening the wall structure.
The seal structure preferably comprises two side walls, a rear wall and a roof portion. The flow means extending from the return duct to the seal channel may be formed in the lower part of the rear wall and/or at least one side wall. In addition to the side walls, even the rear wall and/or the roof portion of the seal structure may be cooled by the water tubes bent from the water tube wall defining the furnace.
The durability of the seal structure walls comprising water tubes may be increased by joining adjacent water tubes to each other by means of refractory material or by narrow metal plates, i.e., fins. Preferably, the water tubes of the walls and the fins between the water tubes are lined with refractory material to increase their wear resistance.
It is possible to bend the water tubes of the water tube wall defining the furnace to extend from the front wall to the side walls, then through the rear wall, or directly, to the roof portion, and finally back to the water tube wall defining the furnace. In this connection, the water tubes bent from the water tube wall also refer to tubes which are continuous with respect to the water flow, but separately bent to a desired form and thereafter, joined through welding to the water tubes in the furnace wall and their water circulation.
Preferably, the horizontal cross section of the seal channel is substantially rectangular, and the width thereof parallel to the first wall defining the furnace is at least approximately 1.5 times the depth perpendicular thereto. The width of the seal channel may be, for instance, two to three times its depth, or even more. The gas seal may also comprise at least two adjacent seal channels parallel to the first wall and in connection with the common return duct. Thereby, the total width of the seal channels is preferably at least about three times their depth. If necessary, the total width of the seal channels may even be equal to the width of the first wall, whereby the bed material circulating from the particle separator can be distributed throughout the whole width of the furnace quite evenly.
It is not necessary to divide the return system for bed material in accordance with the present invention, even if it is a very wide one, into separate sections by means of side walls. Preferably, the seal channel may also form a continuous space, whereby the water tubes bent from the furnace wall are used at the return means, e.g., for establishing the rear wall of the return unit or separate supporting structures for the seal channel. Especially, this kind of a wide seal channel is preferably provided with a number of return means. In some cases, it may be most preferable to use every other tube of the wall to cool and to support the seal structure of the gas seal and leave the rest of the tubes unbent or bend them only in close proximity of the furnace wall so as to form a large number of narrow return means.
The lower part of the return duct in accordance with the invention includes a seal channel of the gas seal and a down leg conducting bed material from the return duct down to the seal channel. These channels may be provided, when seen from the furnace, one after the other or side by side. In some cases, it is preferable to arrange the down leg and the seal channel side by side, as the extent of the lower part of the return duct from the furnace wall can thus be kept small and the supporting of the return duct is easy.
When it is especially important to distribute the recycled bed material evenly throughout the width of the furnace wall, it is advantageous to use several seal channels arranged side by side, when seen from the furnace. These seal channels may cover almost the whole area of the first furnace wall. Thereby, it is advantageous to provide a down leg in the gas seal, which down leg may be common to all seal channels and located subsequent to the seal channels when seen from the furnace.
In large circulating fluidized bed boilers having a plurality of particle separators, it is also natural to have several return ducts provided with gas seal arrangements. It is also possible to collect the material recycled from two separators in one return duct or to divide the material separated in one separator to flow into two return ducts, of which, for instance, only one leads to a separate heat exchange chamber. It is possible to apply the present invention to all these cases, thus effecting an even distribution of the material recycled to the furnace and keeping the bearing capacity of the furnace wall constant.
The return duct is preferably formed of planar water tube panels. Thus, one of the water tube walls forming the return duct may preferably be a section of the water tube wall defining the furnace. When using a gas seal structure in accordance with the present invention, the whole return duct may form an integrated unit with the furnace wall. The extension of the return duct wall on the furnace side may also form the rear wall of the seal channel, whereby the seal channel may be at least partially disposed between the extension of the return duct wall on the furnace side and the first wall defining the furnace.
The horizontal cross section of the lower part of the return duct is preferably rectangular and its width in the direction of the first wall is at least approximately twice the depth perpendicular thereto. The width of the cross section may preferably be, for instance, three or four times its depth, or even more.
The front wall of the seal channel in the gas seal is preferably shared by the furnace. The front wall may be a water tube structure provided with refractory lining, an uncooled metal structure lined with refractory material or a simple structure of refractory material. According to the present invention, at least one wall of the seal channel is preferably a water tube structure provided with refractory lining. The other walls of the seal channel may be refractory material provided with water tube structures, comparable metal structures or simple structures of refractory material.
A gas seal in accordance with the present invention preferably comprises at least two adjacent seal channels in communication with a common return duct. Adjacent seal channels may be totally independent or they may share common partition walls or form a space which is not divided at its upper and/or lower end. A seal channel may have side walls of its own, or the side walls of the lower part of the return duct may also partially act as side walls of the seal channel.
By applying the present invention, it is possible to provide a gas seal in connection with the furnace wall in such a way that the wall remains efficiently cooled and maintains its durability and may thus also act as a supporting structure in the furnace.
When the gas seal of the fluidized bed reactor is formed in connection with the cooled furnace wall without thick refractory linings, the outside dimensions of the gas seal are minimized and the weight of the gas seal remains moderate. Thus, the gas seal may be supported economically without large and expensive supporting structures. A cooled gas seal in accordance with the present invention is also durable and its temperature can be changed relatively quickly, for example, during start-ups and shutdowns without any damage to its structure.
The inner dimension of the seal channel cross section parallel to the front wall, i.e., the width, is larger, most preferably at least 1.5 times larger, than the inner dimension perpendicular thereto, i.e., the depth of the seal channel. When using an uncooled front and/or rear wall in the seal channel, the width measured in the direction of the furnace wall is to be quite small, preferably less than approximately 1000 mm, most preferably 300-500 mm. When using cooled front and rear walls, the width of the seal channel may be increased also by arranging local cooling, for example, in the middle of an otherwise uncooled wall. The width of the seal channel needs to be such that the furnace walls and seal channel walls remain sufficiently cooled and durable in every place.
The idea behind the present invention is that the circulating flow from the particle separator should be distributed evenly by means of a return duct integrated in the furnace wall throughout the whole furnace. The integration of the return duct in the furnace wall is optimized, with respect to space utilization and constructional strength, when the lower part of the return duct and the gas seal arranged therein are wide in the direction of the furnace wall and extend as slightly as possible outwards from the furnace. Thereby, the gas seal may preferably be realized in such a way that the supporting structures thereof are integrated in the supporting structures of the furnace wall.
As for the durability of the structure, it is advantageous to divide the wide gas seal in accordance with the present invention, at least in the area of the opening between the gas seal and the furnace, into compartments by special side walls, which are cooled by the water tubes of the furnace wall bent away from the area of the opening.
There are several methods to manufacture the gas seal in accordance with the invention. It is common to each of them that the pipes in the furnace wall are bent in such a way that openings required for recycling the circulation material are formed in the wall and the tubes bent from the furnace wall are utilized in the structure of the gas seal walls.
According to a first preferred embodiment, the tubes bent from the furnace wall are used primarily to form side walls for the seal channels in the gas seal. Thus, the tubes that are above and below the gas seal, adjacently in the furnace wall, are at the level of the gas seal subsequently in the space between the front wall and the rear wall, whereby the plane they form is at least approximately perpendicular to the furnace wall.
This kind of a structure is simple to manufacture and it may be realized in such a way that the bed material flow in the seal channel is fluent and the bearing capacity of the furnace wall does not substantially decrease. When this structure is used, the rear wall of the seal channel is preferably an uncooled structure provided with refractory lining.
According to another preferred embodiment, the front wall, the side walls and the roof portion are cooled by water tubes bent from the water tube walls of the furnace. By leaving the lower parts of the side walls of the seal channels uncooled or open, it is possible to cool the front wall of the seal channel substantially efficiently throughout its whole area.
According to a third preferred embodiment, tubes of the furnace wall are used for forming a front wall, side walls, a rear wall and a roof portion of the seal channel. When the lower parts of the side walls are left open, it is possible to cool all seal channel walls efficiently by means of the water tubes of the furnace wall.